What causes breast cancer? Millions of dollars and thousands of research hours have been spent searching for the answer, and still that answer eludes us. But research is making great strides focusing on drugs called SERMS (Serum Estrogen Receptor Modulators), that may prevent breast cancer for some women.

Tamoxifen is a derivative of estrogen and the first SERM drug, shown to help prevent breast cancer. In late 1998, researchers announced that Tamoxifen reduced the risk of breast cancer by almost 50% in a study that involved over 13,000 American women at high-risk for the disease. Tamoxifen has been used to treat breast cancer since 1978, and in 1998, it was approved by the U.S. Food and Drug Administration to reduce the incidence of breast cancer in women at high risk. Drugs like Tamoxifen, have a chemical relationship to estrogen, but are altered so that they don't act exactly like estrogen.

So what is the estrogen equation exactly? Estrogens affect the body in a wide variety of beneficial ways. Most commonly, they keep a woman premenopausal, decrease the risk of osteoporosis and also may decrease the risk of heart disease.

For many high risk women the good news about drugs like Tamoxifen outweigh the bad. For example, within a high risk group of women for developing breast cancer, less women will develop the disease if they take Tamoxifen. In other words, if two out of twenty women would develop breast cancer over five years without Tamoxifen, only one of those twenty women will develop breast cancer within that five year period if they were all taking Tamoxifen. That's a 50% decrease in the risk of breast cancer, a figure not to be scoffed at.

Another example, might be a 45 year old woman who has been told she has a 6% chance of developing breast cancer over the next five years. If she were to take Tamoxifen, she would reduce her risk to only 3% over five years. Unfortunately, she would not know whether she was in the 3% that was benefited by Tamoxifen, the 3% that would develop breast cancer despite taking Tamoxifen or the 94% that weren't affected either way. She would also have a small increased risk of blood clots and uterine cancer. However, since she is less than 50 years old, that risk is only about a half a percent, which lessens her overall benefit to 2.5%. This means that one person out of every 40 women taking Tamoxifen in this group would benefit from the drug ­ a significant accomplishment.

Despite all these benefits, Tamoxifen poses some very real risks for women, such as the tendency to cause blood clots more easily in the leg veins, lungs or the brain. Taking Tamoxifen also slightly increases the risk of a woman developing uterine cancer. However, these clots occur very infrequently and mainly if you are over 50 years old. Overall, the total risk of side effects is only about 1% ­ a very small figure.

Still, there are some groups of women for whom taking Tamoxifen won't make much of a difference. For example, if a woman, 60 years old with a 3% risk of breast cancer over the next five years who also has not had a hysterectomy takes Tamoxifen it would decrease her risk of developing breast cancer from 3% to 1.5% (or a 1.5% benefit). However, she would also have the risks of blood clots and uterine cancer for a woman over 50, which would be about 1%. Her net benefit is only 0.5% (1.5% benefit less 1% risk). This means that over the five year period, only one person out of 200 women in this group taking Tamoxifen would benefit from the pills. Not the best odds in this situation.

Clearly, taking Tamoxifen is a matter of carefully thought out choice. Before deciding whether you should take it to prevent breast cancer, you have to determine your own risk factors, which include: 1) Your estimated risk of developing breast cancer over the next five years, 2) Whether you are over or under 50 years old (women over 50 have higher risk of side effects), 3) Whether you have had a hysterectomy (if you've had one you cannot develop uterine cancer).

The final decision depends on how you (and your doctor) individually weigh the risks and benefits of the drug for yourself. As with most important decisions in life, it's all up to you.